Recently, attempts have been made to form superconductive oxide films in the yttrium-barium-copper-oxide system (which is herein below abbreviated as "Y-Ba-Cu-O system"), the bismuth-strontium-calcium-copper-oxide system which is hereinbelow abbreviated as "Bi--Sr--Ca--Cu--O system"), and the thallium-bariumcalcium-copper-oxide system (which is hereinbelow abbreviated as "Tl--Ba--Ca--Cu--O system ). One of the attractive candidates for forming the superconductive oxide film is the co-sputtering technique where a plurality of targets of different substances are installed in the sputtering chamber and simultaneously sputtered to deposit a composite film of a superconductive oxide film.
For example, in order to form a superconductive oxide film in the Y--Ba--Cu--O system on a substrate by using the co-sputtering technique, three targets, namely, a metallic copper target, a Y.sub.2 O.sub.3 target and a BaCuO.sub.2 target are used for depositing the superconductive oxide film on the substrate disposed in facing relationship with the targets, and the composition of the superconductive oxide is adjustable by independently controlling the powers applied between the shield plates and the targets, respectively.
For formation of a superconductive oxide film in the Bi--Sr--Ca--Cu--O system by using the co-sputtering technique, the co-sputtering system requires another set of the targets, i.e., a metallic copper target, a metallic bismuth target and a SrCaCu.sub.2 O.sub.4 target. These targets are subjected to bombardments of, for example, argon ions and, accordingly, sputtered to deposit the superconductive oxide film on a substrate.
In a similar manner, a metallic copper target, a Tl.sub.2 O.sub.3 target and a BaCaCu.sub.2 O.sub.4 target are installed in the sputtering chamber for formation of a superconductive oxide film in the Tl--Ba--Ca--Cu--O system, and these targets are simultaneously sputtered with the bombardments of the argon ions.
The above-mentioned BaCuO.sub.2, SrCaCu.sub.2 O.sub.4 and BaCaCu.sub.2 O.sub.4 targets are produced through the following process sequences. The process sequences start with preparation of powders of barium carbonate, strontium carbonates, the calcium carbonate and copper oxide which are not greater than 10 microns in average diameter.
(1) Production of BaCuO.sub.2 Target:
The powders of the barium carbonate and the copper oxide are regulated to a predetermined proportion and mixed into each other. The mixture is calcined at 850 to 950 degrees in centigrade for 10 hours, and the calcined product is pulverized. The calcination and the pulverization are repeated twice or three times to produce a powder of BaCuO.sub.2. Subsequently, the powder of BaCuO.sub.2 is formed into a target by using a hot-pressing technique under the following conditions:
Atmosphere: a reduced pressure of not greater than 10.sup.-2 torr PA1 Heating temperature: 600 to 900 degrees in centigrade PA1 Compacting pressure: 50 to 300 kgf/cm.sup.2 PA1 Heating time: 2 to 10 hours
(2) Production of SrCaCu.sub.2 O.sub.4 Target:
The powders of strontium carbonate, calcium carbonate and copper oxide are regulated to a predetermined proportion and mixed into one another. Then, the resulting mixture is calcined at 850 to 950 degrees in centigrade for a certain period of time, and the calcined product is pulverized. The calcination and the pulverization are repeated twice or three times to produce a powder of SrCaCu.sub.2 O.sub.4. Subsequently, the powder of SrCaCu.sub.2 O.sub.4 is formed into a target by using the hot-pressing technique under the similar conditions to the BaCuO.sub.2 target.
(3) Production of BaCaCu.sub.2 O.sub.4 Target
The powders of barium carbonate, calcium carbonate and copper oxide are regulated into a predetermined proportion and mixed into one another. Then, the resulting mixture is calcined at 850 to 950 degrees in centigrade for a certain period of time, and the calcined product is pulverized. The calcination and the pulverization are repeated twice or three times to produce a powder of BaCaCu.sub.2 O.sub.4. Subsequently, the powder of BaCaCu.sub.2 O.sub.4 is formed into a target by using the hot-pressing technique under the similar conditions to the BaCuO.sub.2.
However, the prior art targets are merely composed of oxides such as BaCuO.sub.2, SrCaCu.sub.2 O.sub.4 and BaCaCu.sub.2 O.sub.4, respectively, and, for this reason, a problem is encountered in each of the prior art targets in low thermal conductivity. A large difference in temperature takes place between the sputtered surface of the target and the cooled surface thereof exposed to the associated cooling system, so that cracks are liable to be produced in the target due to the thermal stress during the sputtering. The prior art target is thus short in service life, and the superconductive oxide film is large in the production cost.
Furthermore, these targets are too high in resistivity to be used in a DC diode sputtering system, and, accordingly, an RF sputtering system is required to deposit the superconductive oxide film. Since the RF sputtering system is complicated and, accordingly, more expensive than the DC sputtering system, the expensive system gives rise to increase the production cost of the superconductive oxide film.
Moreover, the prior art targets are low in mechanical strength and, accordingly, brittle, so that meticulous care is necessary for handling the prior art target. The prior art target is much liable to be broken due to careless handling and suffers from short service life. This also increases the production cost of the superconductive oxide film.